The present invention relates to a sensor for the detection of gases, and, in particular, to a sensor for the detection of gases wherein an active element is surrounded by a material of specific physical characteristics.
A number of gas sensors or detectors include active elements at which an analyte gas is reacted for detection thereof. Combustible (flammable) gas sensors, for example, have been in use for many years to, among other things, prevent explosive accidents. Gas detectors generally operate by catalytic oxidation of combustible gases. Conventional combustible gas sensors typically include an active element comprising, for example, a platinum wire coil encased in a refractory (for example, alumina) bead, the surface area of which is covered with a catalyst. An active element comprising an encased platinum coil is commonly referred to as a pelement or a pellister. A detailed discussion of pelement and catalytic combustible gas detectors comprising such a pelement is found in Mosely, P. T. and Tofield, B. C., ed., Solid State Gas Sensors, Adams Hilger Press, Bristol, England (1987).
In general, the active element or pelement operates as a miniature calorimeter used to measure the energy liberated upon oxidation of a combustible gas. The platinum wire or coil serves two purposes within the pelement: (1) heating the bead electrically to its operating temperature (typically approximately 500xc2x0 C.) and (2) detecting changes in temperature produced by oxidation of the combustible gas. During operation, the active element is heated to its operating temperature, where it typically catalyzes the oxidation of the combustible gas analyte(s). The heat released by the combustion reactions is detected by the active element as a temperature rise, providing a measure of the amount of combustible gas analyte present in the environment being monitored.
The increase in temperature is typically measured in terms of the variation in resistance of the platinum coil (with temperature variation). In most cases, the catalytically active element is paired with a second, inactive element or compensating element (that is, a reference resistance) for compensation of environmental factors other than combustible gas concentration, such as ambient temperature, humidity, etc. This type of sensor has been described, for example, in U.S. Pat. No. 3,092,799. The change in resistance of the active element is thus measured in relation to the change is resistance of the reference resistance. Preferably, therefore, the reference resistor comprises a compensating, nonactive element matched as closely as possible with the catalytically active element. The two resistances are part of, for example, a Wheatstone bridge circuit. The voltage developed across the circuit when a combustible gas analyte is present provides a measure of the concentration of the combustible gas.
A catalytically active element of a gas sensor can take forms other than a pelement as describe above. For example, sensors based on solid-state semi-conductor technologies have recently been developed for detection of gases. In such gas sensors, the progression of primary oxidation/reduction reaction steps as molecules of analyte gases interact with the semiconductor""s surface causes its conductivity to change. The change in conductivity can be related to the concentration of analyte gases present in the atmosphere being monitored. Like the catalytic sensor, the active element of the semiconductor-based sensor is typically heated to relatively high operating temperature (for example, approximately 500xc2x0 C.).
In portable, battery-powered instruments, minimization of the power consumption of gas sensors is very important to extending battery life. The industry is thus moving toward low-power gas sensors, preferably with operating voltages that match battery voltage. Most often, power reductions are achieved by employing higher resistance heaters, which are generally smaller and more fragile than their low-resistance counterparts. Catalytic beads based on coils of small diameter wire (for high resistance) are especially susceptible to breakage when a portable instrument is dropped or jarred during xe2x80x9cnormalxe2x80x9d use. Approaches to improving the stability of low-power beads against mechanical shock include incorporation of an xe2x80x9cinsulatingxe2x80x9d layer of glass or ceramic wool to protect the elements. See U.S. Pat. No. 5,601,693. Such an insulating layer, however, can result in an increase in the power requirements of the device.
The industry has also been moving toward sensors that are more tolerant to both temporary inhibitors (such as hydrogen sulfide) and permanent poisons (such as silicones). Silicones are a particularly noteworthy class of poisons because of their debilitating effects on conventional combustible gas sensors and their increasing use in environments where combustible gas concentrations are monitored. Efforts to mitigate the effects of silicone-poisoning at the sensor level have centered on the addition of adsorbent (silicone-scavenging) materials to the bead (see U.S. Pat. Nos. 4,111,658 and 4,246,228) and coating the bead with inert layers of porous (silicone blocking) material (see U.S. Pat. No. 4,246,228).
European Patent Application No EP0094863 discloses filling the space around the active element, which is large compared to the volume of the element itself, with a zeolite adsorbent. The zeolite powder, preferably sodium Y zeolite, purportedly protects the catalytic bead from poisoning by silicone compounds without causing a discernible loss in sensitivity. It is also purported that the thermal insulating properties of the zeolite of European Patent Application No EP0094863 are conservative of sensor heat.
Although many improvements have been made in sensors for detecting gases, it remains desirable to develop sensors with improved durability, lower power requirements and/or increased poison resistance.
Generally, the present invention provides a gas sensor for the detection of gases comprising an exterior housing and an active element disposed within a housing. The active element is surrounded by a porous insulating material. Preferably, the porous insulating material has a bulk density of less than 0.3 g/cc. More preferably, the porous insulating material has a bulk density of less than 0.15 g/cc. Most preferably, the porous insulating material has a bulk density of less than 0.1 g/cc. It has been discovered that such low-density, porous materials increase the shock resistance of the sensor while surprisingly and effectively reducing heat losses from the active element.
As used herein in connection with the porous insulating material, the terms xe2x80x9csurroundxe2x80x9d or xe2x80x9csurroundingxe2x80x9d indicate that the element is encased in or encompassed by the porous material such that the gaseous atmosphere to be tested must pass through the porous insulating material to reach the element. The surrounding porous insulating material can be in substantially any form including, for example, in powder form, in flake form, in a blanket form, or formed in place as a monolith. The porous insulating material may also be painted on the active or compensating element. Preferably, the porous insulating material is in powder form.
It has also been discovered that response time or rise time of certain analytes is inversely proportional to the surface area of porous materials surrounding an active element, particularly in the case of a porous materials comprising silica or alumina. It is believed that certain hydrocarbons, (for example, heptane and toluene) may have a weak attraction for the surfaces of materials such as silica and alumina, which can retard diffusion of such hydrocarbons to the active element and, thereby, slow response time of the detection device.
The present invention thus also provides a gas sensor for the detection of combustible gases comprising, a housing and an active element disposed within the housing. The active element is surrounded by a porous material having a surface area less than approximately 200 m2/cc. More preferably, the surface are of the porous material is no greater than 100 m2/cc. Even more preferably, the surface are of the porous material is no greater than 50 m2/cc. Even more preferably, the surface area of the porous material is no greater than approximately 30 m2/cc. Most preferably, the surface area of the porous material is no greater than approximately 20 m2/cc.
The present inventors have further discovered that relatively large average pore size assists in achieving a relatively fast response or rise time, especially for larger hydrocarbons such as heptane and toluene. In that regard, the present invention also provides a gas sensor in which the active element is surrounded by a porous material preferably having an average pore size of at least approximately 100 xc3x85. More preferably, the average pore size is at least approximately 150 xc3x85.
With respect to the tolerance/resistance of the gas sensors to poisoning, and particularly to poisoning by silicone compounds, it has been discovered that the chemical and physical nature of the surface of materials plays a significant role. In general, poison tolerance depends upon the interaction between the poison and the solid surface. For example, the poison may retain its chemical identity while being loosely or moderately bound (xe2x80x9cphysisorbedxe2x80x9d or xe2x80x9cchemisorbedxe2x80x9d) to the solid surface. A poison may also chemically react at a xe2x80x9csitexe2x80x9d on the surface (that is, a specific arrangement of atoms on the solid). xe2x80x9cActivexe2x80x9d surfaces possess chemical groups that interact (via, chemisorption or reaction sites) with poisons.
Silica, for example, has a surface that is substantially inert or inactive with respect to, for example, silicon poisons. An alumina surface is an example of an active surface. Alumina, for example, has been found to be more effective in xe2x80x9ctrappingxe2x80x9d silicone compounds such as hexamethyl disiloxane (HMDS, a model silicone compound) than a silica surface. It is believed that an alumina surface has weak acid sites, week base sites, weakly oxidizing sites and weakly reducing sites that weakly bind with such compounds. Such sites are not believed to be present on silica surfaces. Zeolites have, for example, much stronger acid sites than alumina and can be considered xe2x80x9cmore activexe2x80x9d than alumina.
It has also been discovered that the pore volume of porous materials also has an effect upon the tolerance of the sensor to silicone poisons. In that regard, relatively large pore volumes are preferred. Preferably, the pore volume of a porous material is at least approximately 0.05 cc/cc. More preferably, the pore volume of the porous material is at least approximately 0.10 cc/cc. Surface areas, average pore sizes and pore volumes set forth in the studies of the present invention were determined by nitrogen adsorption/desorption techniques as known in the art.
In addition to the parameters discussed above as affecting tolerance/resistance of the gas sensors to poisoning, chemical compounds can be used to xe2x80x9cscavengexe2x80x9d poisons from a gas sample before the gas sample reaches the active element of a gas sensor. For example, silver-containing compounds can be used as a dopant upon the surface of a porous material to transform inactive sites to active sites. Silver-containing compounds, however, can act as a catalytic material for the reaction/combustion of certain analyte gases. This catalytic activity of silver-containing compounds can result in inaccuracies, particularly when the silver-containing compounds are in the vicinity of or surrounding a compensating element.
The present inventors have discovered that copper-containing compounds improve the tolerance/resistance of gas sensors to a number of poisons, including, for example, sulfur containing compound. As copper-containing compounds are generally not catalytically active compounds for numerous analyte gases, copper-containing compounds do not suffer from the problems associated with silver-containing compounds discussed above. The present invention thus also provides a gas sensor for the detection of gases comprising an exterior housing and an active element disposed within a housing. The gas sensor further provides a copper-containing compound positioned such that a gas sample contacts the copper-containing compound before contacting the active element. As clear to one skilled in the art, the copper containing compound may be positioned either within the exterior housing of the gas sensor or outside the exterior housing. The copper compound is preferably supported upon a porous material as described above. The copper compound is preferably copper sulfate.
In another embodiment, the present invention provides a gas sensor for the detection of an analyte gas comprising, a housing and a heating element disposed within a first chamber in the housing. The heating element is surrounded by a porous material (for example, a porous powder as described above) that supports a catalyst that is suitable to catalyze a reaction of the analyte gas upon heating. In this embodiment, the heating element need not be a catalytically active element.
The gas sensors of the present invention preferably further comprises a compensating element disposed within the housing. The compensating element is preferably closely matched to the active element. In that regard, any material surrounding or in the vicinity of the active element is also preferably surrounding or in the vicinity of the compensating element.
The gas sensors of the present invention thus provides one or more of the following advantages: (1) reduced heat losses/power consumption, (2) improved tolerance to silicone-based and other poisons for longer sensor life and a more stable signal over time, (3) improved mechanical shock resistance of the sensor, and (4) reduced flow-rate dependence of sensor output.